Electrical power in public power grids, or utility grids, is supplied by a variety of energy sources that convert mechanical energy into electrical energy. With ever increasing costs of fossil fuels and possible environmental damage resulting from energy generation using fossil fuels, an increasing fraction of the electrical power supporting public power grids is generated at power plants utilizing renewable energy sources. Wind power plants are an example of such power plants. In a wind power plant or farm, a large number of wind turbines, typically numbering tens or hundreds or more, may be connected to a power grid in order to generate and supply electric power to consumers, which in general are remotely located with respect to the wind turbines. The power generated in the wind turbines is sent through transmission or distribution lines of the power grid to the consumers.
Offshore wind farms, i.e. wind farms located at sea at a distance from the shoreline, is a growing market, for example due to the less visual impact with respect to the surroundings offshore wind farms may have compared to wind farms installed on an onshore location, i.e. located on land, or in the water relatively close to shorelines. Moreover, by locating wind farms offshore, disturbance of residents caused by the generally significant amount of noise generated by the wind turbines can be mitigated or eliminated. Furthermore, locating wind turbines on land may not always be desirable, since in general it is desired that rotor blades of the wind turbine are situated in a laminate flow of air, which requirement may not always be met for onshore wind farms, for example due to presence of various obstacles such as hills, woods, buildings and other structures, etc.
It has hence become increasingly common to locate wind turbine assemblies offshore at a distance from the shoreline where water depths allow the wind turbines to be fixedly attached to a foundation provided at the bottom of the sea or on the seabed. At offshore locations, flow of air is not disturbed by presence of various obstacles such as mentioned above. Furthermore, as mentioned above, offshore localization of wind turbine assemblies may be desired from an aesthetic point of view.
Foundation structures used for accommodating and/or supporting wind turbines offshore may comprise monopile foundations, which can be used for fixedly arranging the towers of wind turbines in an offshore wind farm in relatively shallow-water subsea locations, i.e. on the seabed. Water depths at such shallow-water subsea locations where monopile foundations can feasibly be used typically is between 10 to 40 m.
For many shores, water depth rapidly increases with distance from the shoreline. Fixed foundation structures such as those mentioned above may give rise to prohibitively high costs at large water depths of about 50-80 m. Furthermore, installation of monopile foundations at such large water depths may be associated with technical issues that may be difficult to handle.
To this end, floatable structures for locating wind turbines offshore have been proposed e.g. in documents WO2009/064737 and US2010/0278630. One type of such floatable structures comprises a submerged buoyancy chamber supporting a wind turbine with the tower of the wind turbine arranged on the buoyancy chamber. A part of the submerged buoyancy chamber can be filled with a suitable ballast provided in order to locate the centre of gravity sufficiently below the centre of buoyancy of the submerged part of the structure in order to obtain a stable foundation for the wind turbine. In general, the ballast must be chosen such that the weight of the ballast is so high so as to sufficiently counteract wind and wave forces acting on the wind turbine. Such a floatable structure is typically individually moored to the seabed. However, particularly due to high weight ballast requirements, such floatable structures are in general associated with high costs with respect to among other things installation.